1. Field of the Invention
The present invention relates generally to a suspension control system for controlling vehicular suspension characteristics for achieving both the riding comfort and driving stability. More specifically, the invention relates to a suspension control system, particularly applicable for an active suspension system, which suspension control system performs height adjustment for assuring vehicular driving stability at particular vehicle driving condition inducing inertia forces affecting vehicular attitude.
2. Description of the Background Art
In the modern automotive technologies, active suspension systems have been introduced for better vehicular height and vehicular attitude regulating potential. Such recently developed or proposed active suspension systems generally perform vehicular height regulation and vehicular attitude regulation, e.g. anti-pitching and anti-rolling control.
Japanese Patent First (unexamined) Publications (Tokkai) Showa 62-295714 and 63-235112 disclose typical constructions of active suspension systems. The active suspension system disclosed in the former publication is designed to perform anti-rolling suspension control on the basis of lateral acceleration exerted on the vehicular body. The shown system adjusts fluid pressure in working chambers in a hydraulic cylinder for adjusting a damping force resisting against rolling moment and whereby suppressing vehicular rolling. On the other hand, the latter publication discloses an active suspension system which performs anti-pitching control on the basis of longitudinal acceleration exerted on the vehicular body. The system also adjusts the fluid pressures in the working chambers in the hydraulic cylinders so as to suppress vehicular pitching motion. The latter publication further discloses use of variable gains for controlling front and rear suspension systems, with which gains the longitudinal acceleration indicative signal is amplified to derive front and rear suspension control commands. The variable gain provided in the later publication is expected to provided higher precision and optimum suspension control performance.
In such prior proposed active suspension systems, the fluid pressure in the working chamber is maintained at a predetermined neutral pressure as long as the lateral and/or longitudinal acceleration exerted on the vehicular body is maintained at zero. When the vehicle rolls, the fluid pressures at left and right suspension systems are adjusted by left and right suspension control commands having equal values but having opposite phases or polarities for hardening suspension systems oriented outside of the curve and softening suspension oriented inside of the curve. Similarly, when vehicular pitching is caused, front and rear suspension control commands having equal valve but opposite phases are supplied to the front and rear suspension system for suppressing vehicular pitching motion magnitude.
Theses prior proposed active suspension systems are effective for regulating the vehicular height level substantially at a target height or within a predetermined target height range set about the target height and for regulating vehicular attitude. On the other hand, vehicular driver expect a vehicular attitude change causing lowering of vehicular height at one side of the vehicular body during acceleration, deceleration and cornering. Furthermore, the drivers may feel higher stability if the vehicular height is lowered when inertia force causing vehicular rolling and/or pitching is exerted on the vehicular body. In contrast to this, the active suspension system normally adjusts the vehicular height at least within the target height range even when an inertia force in the longitudinal or lateral direction is exerted. Therefore, despite the fact that the vehicular driving stability is assured by adjustment of suspension characteristics at each of individual suspension systems, the drivers might feel that the vehicular height is excessively high and may bring instability of the vehicle. This may degrade the drive feeling of the vehicle.